DIPOLE INTERACTIONS AND COMPLEX FORMATION 33 



neighbouring molecules are ionised, the forces involved 

 may be rather large, whereas the weakest forces encoun- 

 tered are Van der Waals' forces. Usually the forces which 

 have to be considered can be ranked as follows: ion-ion 

 > ion— dipole > dipole— dipole > Van der Waals' inter- 

 actions. Since different molecular species have different 

 distributions of attractive and repulsive forces in their 

 molecules, at times the interaction between two adjacent 

 molecules may be sufficiently selective to entitle us to 

 regard the interaction as resulting in the formation of a 

 two-dimensional molecular complex (Schulman and 

 Rideal). As a result of such complex formation, changes 

 in at least five variables can be observed. These are: the 

 packing of molecules, the surface pressure, the surface 

 viscosity, the surface potential and the velocity of che- 

 mical reactions in the monolayer. The changes in these 

 variables quite clearly are likely to be of importance as 

 providing possible mechanisms of drug action. 



Chemical reactions at interfaces have recently been 

 examined, with the emergence of some interesting points. 

 For example, the hydrolysis of trilaurin by hydroxyl 

 ions has been shown to be sensitive to surface pressure 

 changes. At a surface pressure of 5.4 dynes per cm the 

 activation energy of the reaction is 10,000 calories, 

 whereas at 16.2 dynes per cm the activation energy has 

 risen to 15,000 calories. The actual orientation of a mol- 

 ecule at the interface may also exercise a profound 

 effect upon the velocity of reaction. For example, with 



